1. FIELD OF THE INVENTION
The present invention relates to a method for cleaning the surface of a spin-on glass layer that has been subjected to an etching procedure. A particular feature of the invention is that it accomplishes the surface cleaning without introducing undesireable side effects such as an increase in the porosity of the spin-on glass layer.
2. DESCRIPTION OF THE PRIOR ART
The general class of compounds known as siloxanes have a chemical structure of the form: ##STR1## where R represents a hydrocarbon radical such as CH.sub.3 or C.sub.2 H.sub.5.
Spin-on glasses are created by dissolving a siloxane in a suitable solvent, such as methyl alcohol, adjusting the concentration of the solution so as to obtain the right viscosity, and then applying the solution to the surface of a semiconductor wafer so as to form a layer of a predetermined thickness. Thickness is controlled by dripping a measured amount of the solution onto the surface of a spinning wafer in the same way that photoresist is applied. The layer of siloxane solution is then allowed to dry, following which it is subjected to heat treatment (approximately 420.degree. C. for approximately 30 minutes) in an nitrogen ambient. During this heat treatment the Si in the siloxane is converted to SiO.sub.2.
Although the polymerization reaction goes to completion for the bulk of the siloxane material, small amounts of unoxidized radicals, R, (typically CH.sub.3) tend to remain behind, trapped inside the SiO.sub.2 film that was created by the Oxidizing heat treatment. Such trapped CH.sub.3 radicals would not necessarily be a problem were it not for the fact that many applications of spin-on glasses in semiconductor technology require that the newly created SiO.sub.2 film (or cured SOG) be etched back by some predetermined amount after it has been created.
Etch-back of the cured SOG layer is generally performed by means of reactive ion etching, typically involving a gaseous atmosphere that contains carbon-fluorine compounds such as CF.sub.4 or CHF.sub.3. As the cured SOG layer is etched away the trapped CH.sub.3 radicals become exposed to etchant gas and some of them react with it to produce a polymer-like material which manifests itself as a thin residue or scum on the surface of the etched-back SOG.
Following the etch back procedure, the next step in the manufacture of the integrated circuit would be to deposit an additional layer of insulating or conductive material onto the surface of the etched-back SOG. If the polymeric residue that resulted from the reaction of the CH.sub.3 radicals with the etchant gas is allowed to remain on the surface of the cured SOG prior to the application of the additional layers, a problem will arise as the presence of the residue will eventually lead to a loss of adhesion between the top surface of the SOG and the layer that has been deposited directly onto it. This loss of adhesion leads to bubbling and/or peeling of all the layers that were deposited onto the etched back surface of the SOG.
A number of methods are currently in use for dealing with this problem, but none of them is completely satisactory:
a) Reactive ion etching in a mixture of CF.sub.4 and oxygen. Only limited improvement is obtained with this method and the microloading effect is enhanced. That is, the SOG etch rate becomes very dependent on the amount of exposed plasma-enhanced oxide at any given time. PA1 b) Raising the temperature at the lower electrode of the etcher. The improvement is slight and etching uniformity is degraded. PA1 c) Oxygen plasma treatment. This method is difficult to control. A slight over-treatment readily results in a cracked and/or porous SOG film in which moisture absorption increases drastically. PA1 d) Argon sputtering. This method leads to a loss in planarity because of the difference in the sputtering rate of the SOG and the plasma enhanced oxide (PE oxide).